This invention relates to air-fuel ratio control device for an internal combustion engines, and more particularly relates to an air-fuel ratio control device for reducing noxious elements contained in the exhaust gas being discharged from the internal combustion engines.
A three-way catalytic converter system is known as a device for diminishing simultaneously three different noxious gases, carbon-monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NO.sub.x), being exhausted from an internal combustion engine. A three-way catalytic convertor is provided in an exhaust pipe and has a characteristic that, when the composition of the exhaust gas being introduced into the three-way catalytic converter is in an extremely narrow range in the vicinity of the composition of the exhaust gas produced by the combustion of the mixture having a stoichiometric air-fuel ratio, the oxidization of CO and HC and the reduction of NO.sub.x are simultaneously performed, which causes simultanious diminishing of these three noxious components. Therefore, if the three-way catalytic converter is used, it is necessary to control the ratio of the amount of air being introduced into the upstream side of the three-way catalytic converter to the amount of fuel being introduced into the upstream side of the three-way catalitic converter (hereinafter refered as the total air-fuel ratio) so that it conforms to the stoichiometric air-fuel ratio. The stoichiometric air-fuel ratio means the air-fuel ratio at which a three-way catalytic converter effectively purifies the three noxious elements, CO, HC and NO.sub.x, simultaneously.
There is a feed-back type air-fuel control system known in the art, which comprises: means for detecting the oxygen concentration of exhaust gas, such as an O.sub.2 sensor provided in an exhaust pipe; a computer means to which the output of the O.sub.2 sensor is input and in which the output voltage is compared with a reference voltage, and; an actuator means for supplying additional fuel or air and controlling the total air-fuel ratio in accordance with the comparing signal from the computor means, so that the total air-fuel ratio is kept in the vicinity of the stoichiometric air-fuel ratio. In these air-fuel control systems, when the O.sub.2 sensor detects a lean air-fuel ratio, the amount of additional fuel is controlled so as to be increased (or the amount of additional air is controlled so as to be decreased), and when the O.sub.2 sensor detects a rich air-fuel ratio, the amount of additional fuel is controlled so as to be decreased (or the amount of additional air is controlled so as to be increased). Therefore, the total air-fuel ratio of the exhaust gas being introduced into the three-way catalytic converter is controlled so as to be maintained in the vicinity of the stoichiometric air-fuel ratio.
However, in a conventional air-fuel ratio control device in which additional fuel is controlled as described above, there are some drawbacks. For example, there is a time delay in response, caused for instance by the means for detecting the oxygen concentration of the exhaust gas, or in other words, a time interval from the changing of the air-fuel ratio of the mixture in an intake system or in an exhaust system to the detecting of the oxygen concentration of the exhaust gas in an exhaust system downstream of the position in which the air-fuel ratio is changed. Consequently, the increasing of the supply of the additional fuel or decreasing of the supply of the additional air does not immediately respond to increase of the suction air when the engine is in the accelerating operating conditions, and thus, the controlled air-fuel ratio is shifted to the leaner side of the stoichiometric air-fuel ratio. During the time the engine is operating in an accelerating condition, the NO.sub.x emission is increased because the amount of suction air is increased. In addition, in an internal combustion engine having an exhaust gas recirculation system, hereinafter referred simply as an EGR system, by which a part of exhaust gas is recirculated into an intake passage in order to reduce NO.sub.x emission, exhaust gas recirculation is performed during the time the air-fuel ratio is lean, which causes a reduction of the operating efficiency of the internal combustion engine.
Some solutions have been proposed for compensating for the gap between a controlled air-fuel ratio and a stoichiometric air-fuel ratio in the prior art air-fuel ratio control systems. Some of these solutions are: a solution in which a proportion constant or an integration constant of the feed back control loop is selected to be a prescribed value in a control circuit (for instance, disclosed in Japanese Laid Open Patent Publication No. Sho. 52-60338); a solution in which the output from a comparator in a control circuit is delayed in accordance with the operating conditions of an engine (for instance, disclosed in Japanese Laid Open Patent Publication No. Sho. 51-146638), and; a solution in which at least one of a proportion constant and an integration constant is changed in a control circuit in accordance with the operating condition of the engine (for instance, disclosed in Japanese Laid Open Patent Publication No. Sho. 51-117231). However, in these air-fuel control devices known from the prior art, it is not easy to keep the controlled air-fuel ratio in the vicinity of the stoichiometric air-fuel ratio, especially during the time the engine is operating in an accelerating condition, which causes a lean air-fuel ratio of the mixture, and thus the efficiency of purification of NO.sub.x contained in exhaust gas is not high.